Delayed coking is one of several types of process used in oil refineries to convert heavy oils to useful lighter products. In delayed cokers, the heavy oil feed is heated in a continuously operating process furnace to effect a limited extent of thermal cracking, after which it enters a large, vertically-oriented cylindrical vessel or coking drum, in which the coking reactions take place. The term “delayed” coker refers to the fact that the coking reactions do not take place in the furnace, but rather are delayed until the oil enters the coke drum. In the coke drum, large oil molecules are further thermally cracked to form additional lighter products and residual coke, which fills the vessel. The lighter hydrocarbons flow out of the drum as vapor and are further processed into fuel products. Gradually the coke accumulates in the drum until it is almost filled with coke. When the drum is nearly filled, the hot oil from the furnace is directed to a clean coke drum, while the full one is decoked. The decoking cycle involves cooling and depressuring the drum, purging it with steam to remove residual hydrocarbon vapor, opening up the top and bottom heads (closures) on the drum and then using high pressure water lances or mechanical cutters to remove the coke from the drum. The coke falls out the bottom of the drum into a pit, where the water is drained off and conveyers take the coke to storage or rail cars. The drum is then closed up and is ready for another coking cycle.
The feedstocks for delayed cokers are typically the heaviest (highest boiling) fractions of crude oil that are separated in the crude fractionation unit, normally comprising an atmospheric distillation tower and vacuum tower. The nature of the coke formed is highly dependent on the characteristics of the feedstock to the coker as well as upon the operating conditions used in the coker. Although the resulting coke is generally thought of as a low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke). Generally, the delayed coker is considered to produce three types of coke that have different values, appearances and properties. Needle coke, sponge coke, and shot coke are the most common. Needle coke is the highest quality of the three varieties which commands a premium price; upon further thermal treatment, needle coke which has high electrical conductivity (and a low coefficient of thermal expansion) is used to make the electrodes in electric arc steel production. It is low in sulfur and metals and is frequently produced from some of the higher quality coker feedstocks that include more aromatic feedstocks such as slurry and decant oils from catalytic crackers and thermal cracking tars. Typically, it is not formed by coking of resid type feeds. Sponge coke, a lower quality coke, is most often formed in refineries from lower quality refinery coker feedstocks having significant amounts of asphaltenes, heteroatoms and metals. If the sulfur and metals content is low enough, sponge coke can be used for the manufacture of anodes for the aluminum industry. If the sulfur and metals content is too high for this purpose, the coke can be used as fuel. The name “sponge coke” comes from its porous, sponge-like appearance. Conventional delayed coking processes, using the vacuum resid feedstocks, will typically produce sponge coke, which is produced as an agglomerated mass that needs an extensive removal process including drilling and water-jet technology.
Shot coke is considered the lowest quality coke. The term “shot coke” comes from its spherical or ovoidal shape ball-like shape, typically in the range of about 1 to about 10 mm diameter. Shot coke, like the other types of coke, has a tendency to agglomerate, especially in admixture with sponge coke, into larger masses, sometimes larger than a foot in diameter. This can cause refinery equipment and processing problems. Shot coke is usually made from the lowest quality high resin-asphaltene feeds and makes a good high sulfur fuel source, particularly for use in cement kilns and steel manufacture. There is also another coke, which is referred to as “transition coke” and refers to a coke having a morphology between that of sponge coke and shot coke. For example, coke that has a mostly sponge-like physical appearance, but with evidence of small shot spheres beginning to form as discrete shapes. The term “transition coke” can also refer to mixtures of shot coke bonded together with sponge coke.
Another type of coke sometimes encountered is generally referred to as “dense coke” by reason of its high density. It results from using very low gravity (heavy) feeds such as those from tar sands and heavy oil crudes such as those from the Orinoco Heavy Oil Belt in Venezuela. These dense cokes are difficult to process: they are hard to cut out of the drum and do not readily form particles which can easily be handled—frequently they form large, heavy, boulder-like lumps. A particular problem is that their density does not make them amenable to quenching in the manner of shot coke or even sponge coke. The surface area of sponge coke makes it possible for the coke to take up water during the quench phase of the cycle so that it cools off relatively uniformly; conversely, the small size of the shot coke particles makes it possible, in principle at least, to quench this product in an acceptably short period of time. If, however, the process has resulted in a combination of coke morphologies in the drum with more than one type of coke product present, the quenching may be non-uniform and eruptions and discharges may occur when the drilling is commenced or the coke discharged through the bottom header. The dense cokes produced from the very heavy oils are particularly troublesome in this respect since their heavy, dense, non-porous nature tends to prevent the quench water from penetrating the coke mass well so that the problems resulting from slow quenching tend to be more frequently encountered, particularly as more and more heavy crude oils are refined to meet demand for fuel products. Unquenched coke presents a particular hazard since it may result in spontaneous coke pit fires and, when loaded onto barges, coke barge fires. This problem is exacerbated by the fact that the heavy oils feeds from the dense cokes produce larger proportions of coke than many other feeds, so aggravating both the extent and severity of the problem.
Since a quenchable coke will cool more evenly than dense, low porous coke morphologies it would be desirable to have the capability to produce a coke product from the heavy oils that can be cooled and quenched in the delayed coker drum, in order to avoid or minimize hot drums and coke fires.